Transient Stability Analysis and Enhancement for VSG with Virtual Impedance based Current Limitation.

Virtual impedance, as a fault current limitation unit, is commonly employed in grid-connected converters with virtual synchronous generator (VSG) control. However, it has been reported in the existing literature that virtual impedance can deteriorate transient stability, and a detailed analysis of the impact of virtual impedance on transient performance is absent. In this paper, a power transfer model of a VSG-based converter considering virtual impedance is newly established. Based on the model, the negative effects of virtual impedance on dynamic performance and transient stability are revealed by using the phasor portrait and attraction region. Specifically speaking, virtual impedance can reduce the maximum transmission power, enlarge power angle overshoot, and shrink transient stability margin. Furthermore, an adaptive transient control scheme is introduced. This method can significantly improve transient performance via control parameters optimization. Detailed optimization of control parameters is presented. Finally, the effectiveness of the theoretical analysis and the proposed scheme are verified by simulations.